postfix, use a custom mysql user table to reject messages

[Andy Yen]

Hi postfixers,
       To send fewer email bounce messages, I would like to reject 
messages at the SMTP session if either the user doesn't exist, or the 
user has exceeded their quota. My mail setup is a bit different in that 
I only use postfix for receiving messages, user accounts and local 
storage is handled completely outside of postfix. What I do have is a 
custom mysql table that lists usernames in my mail system, and whether 
they have exceeded quota or not. I can of course reformat this table or 
duplicate it.

Is there a way to have postfix look in this mysql table, verify the user 
exists and is not over quota, and then reject or accept the message 
within the SMTP session?

Looking through the docs, there doesn't appear to be an out of the box 
way to do this, but perhaps there is a way to take advantage of how 
postfix currently does its user lookup at this stage of message 
processing to achieve this functionality? Or is my only option to go in 
and modify the source code?

Cheers,
Andy